Method and apparatus for producing radioisotopes

ABSTRACT

A method and apparatus for optimizing the production of Pu-238 in a nuclear reactor during normal reactor operation is disclosed wherein the production of Pu-238 is confined to one or more selectively replaced fuel cells with target cells located in the inside of the active volume of the reactor core to maximize the neutron flux for target irradiation. The target cells are modified existing nuclear fuel assembly cells having some fuel rods replaced with target rods of Np-237 forming a cluster array and having rings of water filled rods surrounding the target cluster to produce the desired optimal Pu-238 production.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to the production ofcommercial and medical radioisotopes and more particularly to a methodand apparatus for producing such radioisotopes during normal operationof a commercial nuclear power reactor.

[0003] 2. Description of the Prior Art

[0004] It is generally known that radioisotopes may be produced in anuclear reactor by using some dedicated target tubes located therein.

[0005] By way of example, prior art patents teach various schemes forproducing and using the radioisotopes produced in the nuclear reactor.

[0006] U.S. Pat. No. 4,393,510 teaches that a nuclear reactor may haveflux enhanced Thorium fuel elements blanketed around the active regionof a rector core and between the core barrel and the thermal shield toproduce radioisotopes such as Uranium 233 (U-233) therein. These fuelelements are disposed within the reactor to be removed and reinsertedduring refueling or reactor outage. However, there is no teaching of howto use target tubes and encapsulated target material optimally loadedwithin the reactor to maximize the production of Plutonium 238 (Pu-238)rather than U-233.

[0007] U.S. Pat. No. 4,196,047 teaches the use of specimen holders withencapsulated target material for monitoring nuclear reactors. Again,there is no teaching of such target material being optimally loaded inthe reactor to maximize Pu-238 production.

[0008] U.S. Pat. No. 4,475,948 teaches the use of Lithium Aluminate toproduce Tritium by neutron flux in a nuclear reactor but again fails toteach any optimized method for producing Pu-238.

SUMMARY OF THE INVENTION

[0009] The present invention is drawn to a method and apparatus foroptimizing the production of Pu-238 in a nuclear reactor. To accomplishthis end, the production of Pu-238 is confined to replaced target cellslocated in the inside of the active volume of the reactor core where itcan maximize the neutron flux for target irradiation. The inventionmodifies some existing nuclear fuel assembly by removing some fuel rodsand replacing them with target rods and/or water filled rods to producethe desired target cells. The combination of fuel, target and water rodsis unique in that selective combinations and locations of these rodswithin the target cell assembly tailors the neutron energy spectrum forthe purpose of optimizing the Pu-238 production while minimizing theproduction of other, unwanted irradiation by-products.

[0010] In view of the foregoing it will be seen that one aspect of thepresent invention is to provide a method and apparatus for optimizedPu-238 production in a nuclear reactor.

[0011] Another aspect is to provide the production of Pu-238radioisotope during normal reactor operation periods.

[0012] Still another aspect is to provide an optimized radioisotopeproduction system of replacement isotope target rods for selective fuelrods.

[0013] Still yet another aspect is to provide an optimized radioisotopeproduction system of selectively replaced fuel rods with target rods andwater filled rods.

[0014] These and other aspects of the present invention will be morefully understood after a perusal of the following description of thepreferred embodiment when read in conjunction with the followingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In the drawings:

[0016]FIG. 1. depicts a schematic of a modified 15×15 fuel rod cellhaving a centrally located target rod with concentric rings of waterfilled rods located therearound used to determine the effectiveness ofPu-238 production;

[0017]FIG. 2. is a graph of neutron energies for the FIG. 1 schematic;

[0018]FIG. 3. is a schematic of a 14×14 fuel cell having one type ofcentral target tube configuration surrounded by water filled tubes andfuel rods for optimizing the production of Pu-238 radioisotopes in thetarget tubes;

[0019]FIG. 4. is a schematic of a 15×15 fuel cell having another targettube configuration surrounded by water filled tubes and fuel rods foroptimizing the production of Pu-238 in the target tubes; and

[0020]FIG. 5. is a schematic of a 16×16 fuel cell having five targettube configurations oriented around five guide tubes and surrounded bywater filled tubes and fuel rods for optimizing the production of Pu-238in the target tubes; and

[0021]FIG. 6. is a schematic of a 17×17 fuel cell having another targettube configuration surrounded by water filled tubes and fuel rods foroptimizing the production of Pu-238 in the target tubes.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] Plutonium 238 (Pu-238) is a radioisotope that is produced todayby irradiating Neptunium-Oxide. Pu-238 is different from othercommercially produced isotopes. While most commercially producedradioisotopes are generally used to irradiate food and in medical orindustrial applications, Pu-238 acts as a high energy heat source upondecay and hence is primarily used as a heat source to power devices suchas satellites rather than being used for its radiation qualities.

[0023] The present invention details a method and apparatus foroptimizing the production of Pu-238 in a nuclear reactor by replacing aselect number of centrally located fuel cells with target cell arraysfor producing Pu-238.

[0024] A Pressurized Water Reactor (PWR) usually has between 121 and 241fuel cells all horizontally located in the reactor and each being about14 feet high. Each fuel cell has a series of fuel cells, instrumentationtubes, and guide tubes for movably holding control rods therein tocontrol reactor is output enclosed in a water bath.

[0025] Turning now to FIG. 1, a 15×15 reactor fuel cell array (10) isshown as was configured as a target cell array having a centrallylocated target tube (12) containing Neptunium-Oxide (Np-237) surroundedby water filled tube locations (14), fuel rod tube locations (16) andguide tube locations G.

[0026] A set of Monte Carlo (MCNP) nuclear calculations were performedto demonstrate the potential range of neutron energy “tuning” with theuse of water rods (14) surrounding a target rod (12) in one fuel cellarray contained inside of a commercial nuclear fuel assembly (notshown). For simplicity, a series of ‘concentric rings’ were modeledaround the target rod (12) centrally located inside of one fuelassembly. FIG. 1 depicts the geometric layout of fuel rods, target rod,guide tubes and water rods used in this evaluation. The results of theMCNP calculations are shown in FIG. 2. This plot shows two benefitsresulting from the surrounding of the target rod (12) of Np-237 withwater rods (14). The first is that the number of thermal neutrons can beincreased by a factor of 5 times, when using 0 as opposed to 3 rings ofsurrounding water rods. This will directly increase the Pu-238production rate, which is dependent upon the thermal neutron capturereaction in Np-237.

[0027] The second benefit comes from the decrease in the fast neutronpopulation by a factor of about 1.6 times, between 0 and 3 rings ofsurrounding water rods. This reduces an undesirable (n,2n) reaction inthe Np-237 target material that produces unwanted Pu-236 which isdifficult to separate form the desired product, Pu-238. Thisdemonstrates the benefit of using water rods in a commercial nuclearfuel assembly to tailor the neutron energy spectrum for the purposes ofirradiating target rods to produce radioisotopes, in this case Pu-238from Np-237 thermal neutron absorption.

[0028] Based on the above-described calculations a series of differentconfigurations for target rod assemblies (20, 20′, 20″, 20′″) forselective fuel cell replacement are given as examples for optimizingPu-238 production. They are shown in FIGS. 3-6 and comprise differentclusters of target rods (T) surrounded by various numbers of waterfilled rod cells (shown blank) interspersed among the guide tubes (G)and surrounded by peripherally located fuel rods (F).

[0029] The FIG. 3. target rod assembly (20) has a cluster of target rodsT centrally located around the central instrumentation tube (I) and therectangularly located central guide tubes (G). There is at least threerings of water filled tubes around this cluster ending with peripherallylocated fuel rods (F) as shown.

[0030] It will be understood that more than one of the above describedtarget cell assemblies (20) may be used to replace some of the 121-241fuel cells of the nuclear reactor The same holds for the target cellassemblies (20′, 20″, 20′″).

[0031]FIG. 4. depicts an alternate target cell array (20′) having acluster of target rods T centrally located around the centralinstrumentation tube (IT) and the rectangularly located central guidetubes (G) in a 15×15 fuel cell array. There is at least three rings ofwater filled tubes around this cluster ending with peripherally locatedfuel rods (F) as shown.

[0032]FIG. 5. depicts an alternate target cell array (20″) having acluster of target rods T located around the five guide tubes (G) in a16×16 fuel cell array.

[0033]FIG. 6. depicts yet another alternate target cell array (20′″)comprising a ring of target cells (T) located between a centrallylocated cluster of fuel rods in addition to an array (20′″) of fuel rodslocated around the periphery. There is generally two water filled tubespacings between the ring of target cells and the fuel rods (F).

[0034] It is intended that the above described target cell arrays (20,20′, 20″, 20′″) be used in commercial nuclear power reactors to provideirradiation services for the purpose of producing radioisotopes such asPu-238. In general, the central assembly position (or other positions)in a PWR (Pressurized Water Reactor) or BWR (Boiling Water Reactor) areused to load at least one fuel assembly replacement containing “target”material for producing, by way of neutron or gamma ray irradiation,radioisotopes. These target cell assemblies are loaded and off-loadedduring normal refuel outages and stored in the spent fuel pool. Anothernon-irradiated target assembly, previously prepared, is loaded into thecore for its irradiation cycle. After the refuel outage ends, the targetrods can be removed from the target assembly, packed and shippedoff-site for processing. The target assembly can be set up for its nextirradiation period and stored until the next refuel outage when it getsloaded back into the core. Similarly, the irradiated target assemblycould be off-loaded, have its target cells exchanged with unirradiatedtarget cells, during the same refuel outage and reloaded into the corefor its next cycle of irradiation. This method does not preclude othercore locations, such as peripheral locations from being used.

[0035] As was described, the apparatus is comprised of nuclear fuelrods, control rod guide tubes, water rods and target rods. Thearrangement (axially and radially) and numbers of each of these rods canbe varied so as to tailor the neutron energy spectrum desired for aparticular radioisotope production run. Water rods are used instead of“water holes” to preserve the assembly's thermal-hydraulic performance(i.e.; pressure drop and cross flow mixing). The apparatus could be are-constituted spent fuel assembly or a specially designed andfabricated assembly cage loaded with the required types and number offuel, water and target rods. The guide tube positions can not be changedbecause they must allow for a control rod cluster to be inserted intothe assembly. The target rods could be single or multi-piece.

[0036] This invention provides additional revenue for commercial nuclearpower reactor operators, a new sources of radioisotopes to industry andthe US government agencies (i.e.; NASA, Food Irradiators), and providesa new product or engineering service for operators and cell providers.

[0037] Certain modifications and additions to this disclose will beobvious to those of ordinary skill in this art area. They have beendeleted herein for the sake of conciseness and readability but areintended to fall within the scope of the following claims.

We claim:
 1. A method of optimizing the production of radioisotopes in anuclear reactor having a series of fuel cells comprising the steps of:providing a target cell array having a target rod surrounded by waterfilled rods located inside a series of peripherally located fuel rods;replacing a fuel cell of the reactor with the target cell array duringthe reactor shut down or refueling cycle; and removing the target cellarray during the next reactor shutdown or refueling cycle.
 2. A methodas set forth in claim 1 wherein a series of target rods are formed intoa cluster of target rods.
 3. A method as set forth in claim 2 wherein aplurality of target rod clusters are located in various areas of thearray all surrounded by a circle of water filled rods.
 4. A method asset forth in claim 2 wherein the cluster of target rods is filled withNp-237 to produce Pu-238 during the normal operating cycle of thereactor.
 5. A method as set forth in claim 1 wherein the cluster ofNp-237 filled target rods is surrounded by at least three circles ofwater filled rods.
 6. A method as set forth in claim 3 wherein theplurality of cluster rods are filled with Np-237 and are each surroundedby at least one circle of water filled rods.
 7. A method as set forth inclaim 6 wherein the plurality of cluster rods filled with Np-237comprises multiple clusters each surrounded by at least one circle ofwater filled rods.
 8. A method as set forth in claim 6 wherein theplurality of cluster rods filled with Np-237 comprises multiple clusterseach surrounded by fuel rods and some water filled rods.
 9. A method asset forth in claim 6 wherein the reactor comprises between 121 and 241fuel cells having between 14×14 to 17×17 cells with one or more of themreplaced with target rod arrays.
 10. A target cell array intended forreplacing a fuel cell of a nuclear reactor for producing radioisotopestherein during a normal operating cycle of the reactor comprising: aseries of target cells located inside the target cell array; a series offuel rods peripherally located along the outer edges of the array toenclose said target cells within the array; and a ring of water filledtubes surrounding the target cells and being surrounded by said fuelrods.
 11. A target cell array as set forth in claim 10 including atleast three rings of water filled tubes around said target tubes.
 12. Atarget cell as set forth in claim 10 wherein said plurality of targettubes are formed into three separate clusters having at least one ringof water filled tubes surrounding each cluster.
 13. A target cell as setforth in claim 10 wherein said plurality of target tubes are formed intofour separate clusters having fuel rods surrounding each cluster andsome water filled tubes surrounding each cluster between said clusterand said fuel rods.
 14. A target cell as set forth in claim 10 whereineach target tube is filled with Np-237 for the production of Pu-238within the target tubes.